There
are some 10 million speciesof
living organisms (mostly insects), and many more extinct ones, so they need to
be classified in a systematic way. In 1753 the Swede Carolus Linnaeus
introduced the binomial nomenclature for naming organisms. This
consists of two parts: a generic name (with a capital letter) and a specific
name (with a small letter), e.g. Panthera
leo (lion) and Panthera tigris
(tiger). This system replaced non-standard common names, and is still in use
today.

A
group of similar organisms is called a taxon, and the science of
classification is called taxonomy. In taxonomy groups are based on
similar physical or molecular properties, and groups are contained within
larger composite groups with no overlap. The smallest group of similar
organisms is the species; closely related species are grouped into genera
(singular genus), genera into families, families into orders,
orders into classes, classes into phyla (singular phylum), and
phyla into kingdoms. So you need to remember KPCOFGS.

This
shows how the seven taxons are used to classify humans. As we go through the
taxon hierarchy from kingdom to species, the groups get smaller and the
animals are more closely related.

Kingdom
Animalia

Phylum

Chordata

Class

Mammalia

Order

Primates

Family

Hominidae

Genus

Homo

Species

sapiens

Sponge

P

Earthworm

P

Insect

P

Fish

P

P

DinosaurE

P

P

Bird

P

P

Mouse

P

P

P

Cat

P

P

P

Elephant

P

P

P

Lemur

P

P

P

P

Monkey

P

P

P

P

Orang-utan

P

P

P

P

Gorilla

P

P

P

P

P

Chimpanzee

P

P

P

P

P

AustralopithecusE

P

P

P

P

P

Homo
HabilisE

P

P

P

P

P

P

Neanderthal
ManE

P

P

P

P

P

P

P

Modern
Human

P

P

P

P

P

P

P

E
= Extinct

This shows the complete classification of some other
species:

Earthworm

Mushroom

Garlic

Kingdom

Phylum

Class

Order

Family

Genus

Species

Animalia

Annelida

Oligochaeta

Terricolae

Lumbricidae

Lumbricus

terrestris

Fungi

Mycota

Basidiomycota

Agaricales

Agaricacae

Agaricus

campestris

Plantae

Angiospermophyta

Monocotyledonea

Liliales

Liliaceae

Allium

sativum

The
aim of taxonomists today is to develop phylogenies, family trees
representing true evolutionary relationships. Historically classification was
based on easily observable structures, and gradually this was extended to
microscopic and electron-microscopic detail. The recent advances in embryology
and molecular biology have given new tools such as patterns of life cycle,
larval development, and gene sequences. These have often led to radically
different phylogenies (e.g. humans should really be the "third
chimpanzee").

Until
the middle of this century, life was divided into two kingdoms, plants and
animals. With the greater understanding gained from new techniques this has
been revised, and modern classifications recognise far more diversity and are
less zoocentric. The classification system used today is that of Whittaker
(1959, modified by Margulis), and contains five kingdoms: prokaryotae, protoctista,
fungi, plantae and animalia. The greatest division now
recognised is not between plants and animals (which are relatively similar),
but between the prokaryotes (cells without nuclei) and eukaryotes (cells with
nuclei). The three "higher" kingdoms are distinguished by their
ecological strategies: absorption (fungi), consumption (animals) and
production (plants).